A valve construction of this type is used as a propellant control valve in rocket engines and must remain in the open position from the beginning of ignition up to shutdown of the engine. It is known from DE 3,609,765 A1, DE 4,221,230 C1 and DE 4,323,846 C1 to form an actuator controlled by a pressure medium for effecting the valve switching, the actuator being constructed as a single or double acting piston-cylinder unit which is spring-loaded in the direction of the valve closing position. In each of the two end positions of the piston-cylinder unit, a locking device, which moves transversely to the piston rod of the piston-cylinder unit, acts to lock the piston-cylinder unit in the respective valve position, even when there is a drop in the control pressure. The locking device is provided with two locking surfaces that are pre-stressed by a spring and act opposite one another. A pressure-controllable multiple piston arrangement, acts to release the respective locked locking surface by overcoming the pre-stress of the spring with a stroke activation of the piston cylinder unit from a form-fitting engagement with the piston rod, while the other locking surface remains engaged with the piston rod under the spring force until it engages in a corresponding recess of the piston rod at the end of stroke of the piston-cylinder unit. Such a valve construction with a pressure-controlled double-locking device acting in opposite directions has a relatively complicated mechanical structure and operation and is relatively sensitive to wear due to the frictional contact between the end-position locking device and the piston rod during the stroke motion, whereby it has limited switching reliability and the danger exists that the locking surfaces and the piston rod will jam in the opening stroke resulting in a blockage of the piston-cylinder unit in a partially opened, intermediate position of the valve.